This study analyzes unevenness in physician Medicaid participation (i.e., the extent to which Medicaid patients in a community are concentrated into the practices of a small number of physicians). The principal goal of the project is to answer the following question: Why is unevenness in physician Medicaid participation much greater in certain communities than in others? The specific aims of the project are: to measure the unevenness in physician Medicaid participation in various communities;to examine the association between unevenness in physician Medicaid participation and a variety of community characteristics (e.g., size and composition of the community population;racial segregation;state Medicaid policies);and to analyze how unevenness in physician Medicaid participation and other community-level characteristics affect the Medicaid participation decisions of individual physicians. The project will implement three methodological advancements over previous research: 1) the use of the Gini index to measure unevenness in Medicaid participation in various communities;2) the use of hierarchical linear modeling techniques to analyze physician Medicaid participation decisions, thereby taking into account the inherently hierarchical set of factors influencing these decisions;and 3) the use of cross-level interactions in these models to examine why various groups of physicians respond differently to the community context in which they make Medicaid participation decisions. The findings of the study will be useful to researchers and policy analysts in both state and federal governments, who will benefit from a better understanding of how community-level characteristics foster unevenness in physician Medicaid participation, and thereby limit the poor population's access to health care. Thus, the proposed research project supports the overarching mission of AHRQ: to improve the quality, safety, efficiency, and effectiveness of health care for all Americans. The specific aims of the proposed project are directly related to one of AHRQ's broad-based goals: to achieve efficiency through wider access to effective health care services and reductions in health care costs. The substance of the research, an analysis of community-level effects on unevenness in physician Medicaid participation, fits into the fourth priority area of AHRQ: Payment and organization. Also, because Medicaid patients are poor and disproportionately from racial and ethnic minority populations, the findings of this research will support AHRQ's ongoing efforts to reduce health disparities.